SEM image acquisition device and SEM image acquisition method

ABSTRACT

An SEM image acquisition device including a scanning signal generation unit configured to rotate a scanning direction of the electron beam to be scanned on the sample and generate a scanning signal to be emitted on a position on the sample corresponding to a same region and same pixels on the sample; a deflection device configured to emit the electron beam on a position on the sample corresponding to the same region and the same pixels on the sample, on the basis of the scanning signal generated by the scanning signal generation unit; a detection and amplification unit configured to detect and amplify a signal from the position on the sample corresponding to the same region and the same pixels on the sample, on which the electron beam was emitted by being deflected by the deflection device; and an image generation unit configured to generate an image from when the position on the sample corresponding to the same region and the same pixels on the sample is irradiated, on the basis of the signal detected and amplified by the detection and amplification unit.

BACKGROUND OF THE INVENTION Technical Field

The present invention relates to an SEM image acquisition device and anSEM image acquisition method whereby an image is acquired by emitting anelectron beam on a sample and detecting electrons emitted or reflectedfrom or absorbed by the sample.

Description of the Related Art

Semiconductor devices have been increasingly miniaturized and thepattern sizes of LSI exposure masks have become smaller. Also, opticalproximity correction (OPC) is used. These developments have resulted inthe shapes of mask patterns becoming extremely complicated.

As such, single dimension inspection in a specific direction such aspattern line width measurement or hole diameter measurement for maskshas become insufficient, and contour extraction is needed to performinspection in two directions such as area measurement, and also carryout exposure simulations.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

When performing measurements in two dimensions, pattern edges(hereinafter referred to as “edges”) exist that have various angulardirections such as circular patterns and inclined patterns with roundedrectangular corners. When such edges are parallel to a scanningdirection of a narrowly focused electron beam, problems occur such assignals becoming reduced and black lines (tailing) appearing due to theinfluence of charging at the edges of the pattern and the like.

To solve these problems, image acquisition methods in which an electronbeam is scanned on a mask from a direction rotated to a predeterminedangle have been conceived. For example, technology has been proposed inwhich a rotated image is acquired by rotating a sample and scanning thesample with an electron beam. In addition, technology has been proposedin which a rotated image is acquired by simply rotating the scanningdirection of the electron beam.

In these methods, synthesis or the like can only be performed aftercutting the image before rotation out of the image after rotation andmatching the fields thereof to match the image before rotation with thescanning range thereof from the acquired rotated image. Otherwise, thefields will differ and synthesis will not be possible.

Additionally, the range that the electron beam is scanned on the maskbefore rotation and the range that the electron beam is scanned on themask after rotation differs exactly the amount of rotation and it isimpossible to cause the emission conditions on the mask to strictlymatch. Consequently the emission conditions (emission ranges) vary.

Furthermore, when acquiring images from different scanning directions(scanning directions of the electron beam), even if the ranges (regions)scanned by the electron beam are cut out and the made the same, thereare fundamental problems such as the position on the mask of the pixelsof each image will differ, matching the position on the mask isimpossible or extremely difficult, and acquiring each of the pixelsignals from the same position on the mask is impossible.

Means of Solving the Problems

According to the present invention, when acquiring images from differentscanning directions (scanning directions of the electron beam), it ispossible to acquire and synthesize images of pixel signals from the sameposition on a mask by scanning an electron beam at pixel correspondenceto acquire and synthesize the images.

An aspect of the present invention is an SEM image acquisition deviceconfigured to acquire an image by emitting an electron beam on a sampleand detecting electrons emitted or reflected from or absorbed by thesample. The SEM image acquisition device includes a scanning signalgeneration unit configured to rotate a scanning direction of theelectron beam to be scanned on the sample and generate a scanning signalto be emitted on a position on the sample corresponding to a same regionand same pixels on the sample; a deflection device configured to emitthe electron beam on a position on the sample corresponding to the sameregion and the same pixels on the sample, on the basis of the scanningsignal generated by the scanning signal generation unit; a detection andamplification unit configured to detect and amplify a signal from theposition on the sample corresponding to the same region and the samepixels on the sample, on which the electron beam was emitted by beingdeflected by the deflection device; and an image generation unitconfigured to generate an image from when the position on the samplecorresponding to the same region and the same pixels on the sample isirradiated, on the basis of the signal detected and amplified by thedetection and amplification unit.

In the aspect of the present invention, the SEM image acquisition devicemay further include a synthesis unit configured to synthesize aplurality of images of the position corresponding to the same region andthe same pixels on the sample from when the sample is scanned fromdifferent rotation directions by the electron beam, on the basis of thesignal detected and amplified by the detection and amplification unit.

In the aspect of the present invention, the synthesis unit may beconfigured to synthesize each of a plurality of images of a samerotation direction and a plurality of images of a plurality of differingrotation directions.

In the aspect of the present invention, the synthesis unit may beconfigured to synthesize a plurality of acquired images by signal (pixelsignal) of the position corresponding to the same region and the samepixels on the sample.

In the aspect of the present invention, the synthesis by signal (pixelsignal) may be integration or averaging by signal (pixel signal).

In the aspect of the present invention, the scanning signal generationunit may be configured to make clock cycles the same, or increase theclock cycles, or decrease the clock cycles; generate, as the scanningsignal, a signal sequentially scanned in a predetermined rotationdirection on positions corresponding to all pixels within the sameregion on the sample; and make total scanning time the same, or increasethe total scanning time, or decrease the total scanning time.

In the aspect of the present invention, the scanning signal generationunit may generate, as the scanning signal, one or more of scanningsignals for which a direction to which scanning direction of theelectron beam is rotated to is at least one selected from the groupconsisting of 0°, (tan⁻¹½°), 45°, (tan⁻¹ 2°), 90°, and a directionobtained by adding or subtracting an integer multiple of 90° thereto,the direction being within a range of 0° to 360° or a range of −180° to180°.

Advantageous Effects of the Invention

With the SEM image acquisition device and the SEM image acquisitionmethod according to the present invention, when acquiring images fromdifferent scanning directions (scanning directions of the electronbeam), images of pixel signals from positions of the same pixels on asample are acquired and synthesized. As a result, situations in whichthe signals become reduced or black lines (tailings) appear due to theinfluence of charging or the shape of the edges of circular patternswhen performing measuring in two dimensions or the like is eliminatedand measuring accuracy is improved.

As such, edges can be extracted with high accuracy from patterns in thesynthesized image, having edges in any direction including circularpatterns, inclined rectangular patterns, and the like. As a result,width measurements, area measurements, contour extraction, and otherprocesses can be carried out on the patterns with high accuracy.

Additionally, conventionally, 60 images acquired in the same scanningdirection were synthesized to obtain a synthesized image (with improvedthe S/N ratio) but, with the SEM image acquisition device and the SEMimage acquisition method according to the present invention, 30 imagesare acquired by dividing into two directions, that is, each of the 45°and −45° scanning directions or 15 images are acquired by dividing intofour directions, that is, each of the 0°, −45°, 45°, and 90° scanningdirections, and then all of the acquired images are synthesized togenerate a synthesized image. This configuration enables the highlyaccurate extraction of edges in any direction from a two-dimensionalpattern; and the highly accurate performance of width measurement, areameasurement, contour extraction, and other processes on edges in anydirection of a two-dimensional pattern in the same amount of time as inthe conventional technology (that is, in this case a total of 60 imagesare synthesized, so the time is the same).

Furthermore, with the SEM image acquisition device and the SEM imageacquisition method according to the present invention, the clocks of thescanning signals are the same. As such, the times to acquire onerotation image can all be set to the same time, and the ranges on thesample where the electron beam is emitted can be set to the same rangeand the same pixels. Note that, as necessary, the interval of the clocksof the scanning signals can be increased or decreased to easily change,that is, increase or decrease, the total image scanning time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram according to an embodiment of thepresent invention;

FIG. 2 is an image acquisition flowchart according to an embodiment ofthe present invention;

FIG. 3 is a diagram for explaining the electron beam scanning directionsand data extraction pixels according to an embodiment of the presentinvention;

FIG. 4 is a diagram for explaining a 45° rotation pixel scan accordingto an embodiment of the present invention;

FIG. 5 is an example of an image data table according to an embodimentof the present invention;

FIG. 6 is a rotation pixel scan flowchart according to an embodiment ofthe present invention;

FIG. 7 is an image synthesis flowchart according to an embodiment of thepresent invention;

FIG. 8A is an example of a synthesized image according to an embodimentof the present invention;

FIG. 8B is an example of a synthesized image according to an embodimentof the present invention; and

FIG. 8C is an example of a synthesized image according to an embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

FIG. 1 illustrates a configuration diagram according to a firstembodiment.

An SEM 1 illustrated in FIG. 1 is a scanning electron microscope andincludes an electron gun that generates an electron beam accelerated ata predetermined acceleration voltage, a blanking electrode that deflectsthe electron beam generated by the electron gun so as to block or allowthe electron beam to pass, a condenser lens that converges the electronbeam, and an objective lens that narrowly focuses the converged electronbeam and emits the electron beam on a sample 4.

A scan rotation unit 2 rotates an emission direction of the electronbeam 21 on the sample 4. The scan rotation unit 2 includes a deflectiondevice 22 and a scanning signal generation unit 23.

The deflection device 22 scans (plane scanning in the X-direction andthe Y-direction) the electron beam 21, which is narrowly focused by theobjective lens, on the surface of the sample 4. The deflection device 22includes an electrostatic deflection electrode or an electromagneticdeflector.

The scanning signal generation unit 23 generates scanning signals thatrotate the emission direction, with respect to the deflection device 22,of the electron beam 21 on the sample 4. Additionally, the scanningsignal generation unit 23 generates signals that scan the same regionand the same pixels on the sample from a predetermined rotationdirection (described later using FIGS. 2 to 8).

A sample chamber 3 is a vacuum-evacuable container in which the sample 4or the like is stored.

The sample 4 is an object on which the electron beam 21 is emitted andis, for example, a mask, a wafer, or the like to be measured of apattern.

A stage 5 is a moving stand that is movable in the X-direction, theY-direction, and the Z-direction. The sample (mask) 4 is mounted andsecured on the stage 5. The position of the stage 5 is measuredprecisely in real time using a laser interferometer (not illustrated).

A signal acquisition unit 6 acquires signals by detecting and amplifyingthe secondary electrons and reflected electrons emitted from and alsothe electrons absorbed by the sample 4 with the detection andamplification unit 61 when the electron beam 21 is scanned on thesurface of the sample 4. The signal acquisition unit 6 is, for example,a micro-channel plate (MCP).

A computer 7 is a personal computer that executes various controls andprocesses in accordance with programs. In the present embodiment, thecomputer 7 includes an image generation unit 8, an image synthesis unit9, a contour extraction unit 10, a measuring unit 11, a display device12, and an image data table 13.

The image generation unit 8 sequentially associates signals(one-dimensional signals), acquired by the signal acquisition unit 6,from positions corresponding to the same region and the same pixels onthe sample 4 from when the electron beam 21 was scanned on the sample 4at the predetermined rotation direction, with signals of pixels of theimage in the scanning direction (two-dimensional signals), and generates(restores) images (described later with reference to FIG. 3).

The image synthesis unit 9 synthesizes the plurality of images, whichwere scanned in the predetermined rotation direction, generated(restored) by the image generation unit 8. Specifically, the imagesynthesis unit 9 synthesizes, by integrating or averaging by pixels, aplurality of images of the same rotation direction and a plurality ofimages of differing rotation directions (described later with referenceto FIGS. 2 to 8).

The contour extraction unit 10 is a typical unit for extracting thecontours (edges) of a pattern from the images synthesized by the imagesynthesis unit 9 (e.g. see hereinafter described FIGS. 8B and 8C).

The measuring unit (measurement unit) 11 is a typical unit for measuringthe width, area, and the like of a pattern.

The display device 12 displays images and the like and, in this case, isa display.

The image data table 13 is a table in which the image information,images, and the like of the present embodiment are stored (see FIG. 5).

Next, operations of the constituents illustrated in FIG. 1 will bedescribed in detail with reference to the flowchart of FIG. 2.

FIG. 2 illustrates an image acquisition flowchart according to thepresent embodiment.

In step S1, the number of times n for changing the rotation angle of thescan is set. As indicated on the right side in FIG. 2, n is set to 2times, 4 times, or other. In the present embodiment, n is set to 4times. More specifically, in step S1, the number of times n for changingthe direction (rotation angle) that the electron beam 21 is scanned onthe sample 4 is set. For example, in the case of FIG. 8B (describedlater), n is set to 2 times (the rotation angle is changed to 45° and−45°, therefore n=2 times), and in the case of FIG. 8C, n is set to 4times (the rotation angle is changed to 45°, −45°, 0°, and 90°,therefore n=4 times).

In step S2, the scan is rotated to the setting angle. Specifically,since n was set to 4 times in step S1, the scan is sequentially set toone of 45°, −45°, 0°, or 90° for each repetition of the scan.

In step S3, data is acquired. In cases where, for example, the firstsetting angle set in step S2 is 45°, signals (scanning signals) to besequentially scanned on the pixels are generated having the direction(upward and to the right in the direction of 45°) indicated by thedashed arrows of the (b) 45° scan in the upper left of FIG. 3 (describedlater) as the scanning direction. The scanning of the electron beam 21of FIG. 1 on the sample 4 upward and to the right in the direction of45° (indicated by the dashed arrows of the (b) 45° scan in FIG. 3) isrepeated for all the pixels in the rectangular region on the sample 4 onthe basis of the scanning signals. Secondary electrons (reflectedelectrons, absorbed electrons) emitted when scanning are detected andamplified and acquired by the signal acquisition unit 6 of FIG. 1.

In step S4, the computer acquires an image. Specifically, the signalshaving the 45° scanning direction acquired in step S3 for all of thepixels in the rectangular region on the sample 4 are sequentiallypositioned as signals of positions of the corresponding pixels of FIG.3, and an image is generated (the original image is restored). That is,an image is generated (restored) by associating the one-dimensionalsignals with the signals of the pixels of the two-dimensional image.

In step S5, it is determined if processing has been performed n times.That is, it is determined if the processing for the n times set in stepS1 (4 times in this case) has been completed. If “NO” is determined,step S2 is executed and subsequent processing is performed. On the otherhand, if “YES” is determined, step S6 is executed since the processingfor all n times (here, n=4 times) has been completed.

In step S6, all of the images are integrated. Specifically, for example,when n is set to 4 times in step S1, step S2 to step S4 are repeated 4times and images scanned from the 45°, −45°, 0°, and 90° directions areacquired. Accordingly, in step S6 the images of the four sets areintegrated (or averaged) by pixel correspondence to generate a singlesynthesized image. Note that, a total of 60 images were synthesized inthe experiment. Of these:

15 images were acquired at 45°

15 images were acquired at −45°

15 images were acquired at 0° and

15 images were acquired at 90.°

These 60 images were integrated by pixel correspondence in step S6 togenerate the image shown in FIG. 8C (described later). Note that whenthe number of gradations is excessive in the integration processing bypixel correspondence, a desired number of upper bits after theintegration may be extracted and used. In such a case, lower bits arediscarded.

In step S7, measurement is performed. Specifically, measurement (e.g.measurement of the width size, area, or the like of the rectangle) isperformed using the synthesized image obtained by integrating all of thepixels in step S6.

As described above, the number of times n for changing the rotationangle of the scan can be set as desired and, as a result, it is possiblesequentially change the scan rotation angle and scan to acquire images,and acquire synthesized images from a plurality of scanning directionsby synthesizing these images.

As a result:

in, for example, cases where the scanning direction is a singledirection as illustrated in the image of FIG. 8A (described later), thelines of the pattern parallel to the scanning direction appear to narrowand darken due to the influence of charging, the edge shape, and thelike, butin, for example, the image illustrated in FIG. 8C that was synthesizedafter scanning in the four directions of 45°, −45°, 0°, and 90°described above, in contrast to FIG. 8A, there is no fading of thelateral lines in the pattern and the edges are sharp from alldirections. As a result, it is possible to measure the width, spacing,and the like of the edges from all directions and calculate the area ofthe pattern with high accuracy, perform contour extraction for exposuresimulations, and the like.

FIG. 3 is a diagram explaining the electron beam scanning directions anddata extraction pixels of the present embodiment.

In FIG. 3, examples of the following are schematically illustrated:

-   -   (a) is a 0° scan    -   (b) is a 45° scan    -   (c) is a −45° scan    -   (d) is a 90° scan    -   (e) is a −90° scan    -   (f) is a 135° scan and    -   (g) is a −135° scan.        Note that the electron beam scan region is the same region (same        range) in the illustrated rectangle (square), each small square        corresponds to a pixel, and the electron beam 21 sequentially        scans (digitally scans) across the center position of each of        the small squares.

As described above, the same region and the same pixels are defined forall of the scanning directions, and scanning signals are generated thatsequentially scan each pixel in the illustrated scanning direction atthe same clock. Moreover, the deflection device deflects the electronbeam 21 such that the electron beam 21 is scanned on the sample 4 on thebasis of the generated scanning signals. This configuration enables thesequential scanning of the pixels (positions) on the sample 4 in thescanning directions depicted in FIG. 3.

The scans of (a) to (g) in FIG. 3 are integer multiples of 0° and 45°scans, and are expressed as being within a range of −180° to 180° (orwithin a range of 0° to 360°). This concept is described in simple termsbelow.

(1) When one of x and y is a 0 pixel and the other is a 1 pixeldirection rotation, the scanning directions are the following integermultiples of 0° and 45°: −180°, −135°, −90°, −45°, 0°, 45°, 90°, 135°,and 180° (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315°, and 360°) and arewithin the range of −180° (0°) to 180° (360°).

(2) When one of x and y is a 1 pixel and the other is a 2 pixeldirection rotation, the scanning directions are the following directionsobtained by adding or subtracting integer multiples of 90° to or fromabout 26° (more accurately tan⁻¹½) and about 63° (more accurately tan⁻¹2): . . . −63°, −26°, 26°, 63°, and so on (26°, 63°, and so on) and arewithin a range of −180° to 180° (0° to 360°).

(3) Likewise, when one of x and y is a (n−1) pixel and the other is a npixel direction rotation, the scanning directions are directionsobtained by adding or subtracting integer multiples of 90° to or fromtan⁻¹(n−1)/n and, tan⁻¹n/(n−1), and are angles within a range of)−180°(0° to) 180° (360° (specific values are omitted).

FIG. 4 is a diagram explaining a 45° rotation pixel scan of the presentembodiment. Specifically, FIG. 4 schematically illustrates details ofthe 45° scan depicted in (b) of FIG. 3.

The (a) 0° rotation pixel scan in FIG. 4 corresponds to the (a) 0° scanin FIG. 3. In the (a) 0° rotation pixel scan, each pixel is sequentiallyscanned horizontally to the right, at a 1 pixel scan interval of 1(horizontal and vertical intervals of each pixel are defined as 1), anda 1 line interval of 1.

Accordingly, 1 pixel scan interval×1 line interval=1.

The (b) 45° rotation pixel scan corresponds to the (b) 45° scan in FIG.3. In the (b) 45° rotation pixel scan, each pixel is sequentiallyscanned upward to the right in the direction of 45°, at a 1 pixel scaninterval of √2 (or approximately 1.4), and a 1 line interval of 1/√2 (orapproximately 0.7).

Accordingly, 1 pixel scan interval×1 line interval=√2×1/√2=1, which isthe same scanning time as the 0° rotation pixel scan. As a result, thetotal time to acquire 1 image is the same.

Implementing the settings described above, generating each scanningsignals at the same clock interval, and setting (restoring) the each ofthe signals acquired by sequentially scanning the positions by pixelcorrespondence on the sample 4 to the signals of the positions of theoriginal pixels enable the generation of the 0° rotation image and the45° rotation image in the same amount of time. Moreover, a synthesizedimage can be generated by synthesizing (e.g. integrating; see step S6 ofFIG. 2) both generated images by pixel correspondence.

When moving from the pixel of an end point to the pixel of a start pointwhen scanning in the predetermined rotation direction, pulse voltage isapplied to the blanking electrode to perform what is typically known asblanking. As a result, the electron beam is not emitted on a path fromthe position (pixel) of the end point to the position (pixel) of thestart point of the sample 4 while the electron beam moves along thispath.

FIG. 5 illustrates an example of an image data table of the presentembodiment. Specifically, the information set in the flowchart of FIG. 2and the following various preset information are stored in the imagedata table.

Rotation angle: The rotation angle determined by n, which is set in stepS1 of FIG. 2;

Image data: Image data acquired at the rotation angle which, asdescribed below the image data table of FIG. 3 includes:

-   -   (1 pixel of 8-bit data) m times;    -   60 times/4=15 times integration: A case where all of the images        (60 images) are synthesized by integration when the total number        of images is 60, wherein scans are performed in the four        directions of 0°, −45°, 45°, and 90°, and 15 images are acquired        for each direction.

Data groups: In the preceding example, an image group of 60 images.

-   -   Imaging conditions: Imaging conditions for capturing the images.        Specifically, information such as acceleration voltage, current,        magnification, and sample name.    -   Correction coefficients: Correction coefficients such as height        correction and magnification differences.

Other:

As described above, the images acquired from the plurality of scanningdirections or the synthesized image, and the rotation angle, imagingconditions, correction coefficients, and other information relatedthereto is recorded and stored in the image data table 13. As such, itis possible to easily perform measurements (e.g. measurements ofdimensions and area) of patterns in a sample image by referencing theimage data table 13.

FIG. 6 illustrates a rotation pixel scan flowchart of the presentembodiment. Specifically, FIG. 6 is a flowchart explaining the detailsof scanning the same region and the same pixels in the predeterminedscanning direction described above.

In FIG. 6, in step S11, a 0° direction image is acquired for each pixel.Specifically, in FIG. 3 described above, one image (one frame) of the(a) 0° scan is acquired by sequentially scanning from top to bottom inthe direction of the (a) 0° scan (from the left edge to the right edge),detecting and amplifying to acquire the signals of each of the pixels atthat time, and setting those signals in the memory as signals(brightness signals, normally 8-bit) of each illustrated correspondingposition (pixel). Typically, there is a total of 60 images and, in thiscase, images of two scanning directions are acquired, namely the (a) 0°scan and the (b) 45° scan. Therefore, the total 60 images is divided bytwo and 30 images of the (a) 0° scan are acquired.

Next, one image of the (b) 45° scan is acquired by executing steps S12to S17.

In step S12, the start point and the end point are calculated.Specifically, the start point and the end point of the scanning rangeare set as follows for the first line 1:

-   -   Start point (x0,y0) of scanning range    -   End point (x0,y0) of scanning range.        Note that, in this case, for the sake of ease of understanding,        coordinates of the centers of the pixels represented by the        small squares in FIG. 3, namely (xn+Δx/2, yn+Δy/2, where n=0 to        n), are represented as (xn,yn). Additionally, Δx and Δy are        respectively the X-direction and Y-direction sizes (widths) of        the pixels represented as small squares.

In the example of step S12, the start point and the end point are thesame, so one point is scanned.

In step S13, images are acquired by scanning. Specifically, the rangefrom the start point to the end point set in step S12 is scanned pixelby pixel and signals (typically detected and amplified 8-bit brightnesssignals) from each of the pixels are acquired.

In steps S12 and S13, the signal of the pixel determined by the startpoint and the end point of line 1 of the (b) 45° scan (one pixel in thiscase) is acquired.

Likewise, in steps S14 and S15, the signals of the pixels determined bythe start point and the end point of line 2 of the (b) 45° scan (twopixels in this case) are acquired.

Likewise, the signals of the pixels determined by the start point andthe end point of line (n−1) from line 3 of the (b) 45° scan (threepixels and so on in this case) are acquired.

Likewise, in steps S16 and S17, the signal of the pixel determined bythe start point and the end point of line n of the (b) 45° scan (thelast pixel in this case) is acquired.

By executing the steps S12 to S17, the pixels of line 1 to line n of the(b) 45° scan are each acquired and one image is generated. Thisprocessing is repeated to acquire 30 images of the (b) 45° scan.

Thus, 30 images of the (a) 0° scan are acquired in step S11 and 30images of the (b) 45° scan are acquired in steps S12 to S17. Byintegrating (or averaging) the pixels of all of these images, onesynthesized image can be generated.

Note that in the present embodiment, the scanning speed is set asfollows, but is not limited thereto. Typically, isochronous scanning(scanning using the same time interval clock) is used so that the timeto acquire one image is the same regardless of the rotation direction ofthe scanning.

-   -   Isochronous scanning (scanning using the same time interval        clock): For example, in the case of the (b) 45° scan described        in steps S12 to S17, scanning is performed at a speed of √2        times and the scan line interval to be scanned is 1/√2 times.        Therefore, the scanning time of all of the pixels is the same.    -   Constant speed scanning (scanning at the same scanning speed):        Increases or decreases the integration time. For example, in the        case of the (b) 45° scan described in steps S12 to S17, the        integration time increases √2 times (the clock interval        increases √2 times).

FIG. 7 illustrates an image synthesis flowchart of the presentembodiment.

In FIG. 7, in step S21, the first (and second and so on) image data isimported.

In step S22, it is determined if the image data is OK for synthesis.Specifically, it is determined if noise in the images captured in stepS21 is below a threshold, and if there is correlation between the images(other images, all other images). If YES is determined, step S23 isexecuted. If NO is determined, synthesis is determined not to be madeand canceled and step S21 is executed and the next image data isimported.

In step S23, synthesis (addition and correction) is performed.Specifically, the images imported in step S21 and determined as YES instep S22 are synthesized, that is, the brightness of each image isintegrated or averaged by pixel. For example, in the case ofsynthesizing the 60 images described above, when one image is an 8-bitrepresentation, a region is secured in which there is no overflow whenintegrating 60 8-bit images (at least a 14-bit and typically a 2-byteregion), and integration is performed. Note that when averaging, thenecessary number of bits (e.g. 8 bits) are extracted from the upperpredetermined bits so as to prevent increases in processing time.

In step S24, it is determined if the synthesis has completed.Specifically, it is determined if the synthesis of step S23 has beencompleted for all of the images to be synthesized (e.g. in the examplegiven above, all of the 60 images). If YES is determined, the imagesynthesis is ended at step S25. Thus, one synthesized image is generated(see FIGS. 8B and 8C).

The process described above enables the generation of one synthesizedimage, which is obtained by sequentially integrating, by pixel, all ofthe images scanned in the plurality of directions.

FIGS. 8A to 8C illustrate examples of acquired images of the presentembodiment.

FIG. 8A illustrates an example of a synthesized image of scans in onedirection, namely the 0° scan; FIG. 8B illustrates an example of asynthesized image of scans in two directions, namely the 45° scan andthe −45° scan; and FIG. 8C illustrates an example of a synthesized imageof scans in four directions, namely the 45° scan, the −45° scan, the −0°scan, and the 90° scan. All of the synthesized images were synthesizedfrom 60 images. FIG. 8A was synthesized from 60 0° scan images; FIG. 8Bwas synthesized from 30 45° scan images and 30 −45° scan images, for atotal of 60 images; and FIG. 8C was synthesized from 15 45° scan images,15 −45° scan images, 15 0° scan images, and 15 90° scan images, for atotal of 60 images.

The synthesized image of the 0° scans illustrated in FIG. 8A wassynthesized from scans in one direction. As such, in the sample, thelines on the top and bottom edges of the rectangular patternssubstantially parallel to the direction of the 0° scanning were affectedby charging, the shape of the line ends, and the like, which caused thelines to fade and dark lines (tailing) to appear. This resulted in thesynthesized image being unclear.

On the other hand, the synthesized image of the 45° scans and the −45°scans illustrated in FIG. 8B was synthesized from scans in twodirections. Specifically, FIG. 8B was synthesized from 30 imagesacquired from 45° scans from the lower left to the upper right and 30images acquired from −45° scans from the upper left to the lower right.As such, the lines of the top and bottom edges of the rectangularpatterns in FIG. 8B appeared clearer than in FIG. 8A, in which the topand bottom edges of the rectangular patterns were unclear.

Furthermore, the synthesized image of the 45° scans, the −45° scans, the0° scans, and the 90° scans illustrated in FIG. 8C was synthesized fromscans in four directions. Specifically, a synthesized image (integratedimage) identical to that illustrated by FIG. 8B was obtained from thescans in two directions, namely the 45° scans from the lower left to theupper right and the −45° scans from the upper left to the lower right,and the scans in two more directions, namely the 0° scans from left toright and the 90° scans from bottom to top were added to generate asynthesized image.

As a result, the lines of the top and bottom edges of the rectangularpatterns in FIG. 8C appeared clearer than in FIG. 8A, in which the topand bottom edges of the rectangular patterns were unclear and, while notillustrated in FIG. 8C, when viewing the rectangular patterns in FIG. 8Cat a 45° incline, the lack of clarity in the top and bottom edges of therectangular patterns disappeared, resulting in an extremely clear image.

Thus, the inventors discovered that in cases where the edges of therectangular patterns on the screen are horizontal and perpendicular,clear images can be obtained from scans in two directions and scans infour directions as respectively illustrated in FIGS. 8B and 8C; and incases where the edges of the rectangular patterns on the screen arehorizontal and perpendicular, and also rectangular patterns inclined at45° are present (or circular, elliptical or other patterns are present),clear images can be obtained from the scans in four directions asillustrated in FIG. 8C.

REFERENCE SIGNS

-   -   1 SEM (Scanning Electron Microscope)    -   2 Scan rotation unit    -   21 Electron beam    -   3 Sample chamber    -   4 Sample    -   5 Stage    -   5 Signal acquisition unit    -   7 Computer (PC)    -   8 Image generation unit    -   9 Image synthesis unit    -   10 Contour extraction unit    -   11 Measuring unit    -   12 Display device    -   13 Image data table

What is claimed is:
 1. An SEM image acquisition method for acquiring animage by emitting an electron beam on a sample and detecting electronsemitted or reflected from or absorbed by the sample; the SEM imageacquisition method comprising: rotating a scanning direction of theelectron beam to be scanned on the sample; generating a scanning signalto be emitted on a position on the sample corresponding to a same regionand same pixels on the sample as the electron beam; emitting theelectron beam on a position on the sample corresponding to the sameregion and the same pixels on the sample, in response to the scanningsignal; detecting and amplifying a signal from the position on thesample corresponding to the same region and the same pixels on thesample, on which the electron beam being emitted and deflected; andgenerating an image from when the position on the sample correspondingto the same region and the same pixels on the sample is irradiated, onthe basis of the signal being detected and amplified; wherein thegenerating the scanning signal comprises: making clock cycles the same,or increasing the clock cycles, or decreasing the clock cycles;generating, as the scanning signal, a signal sequentially scanned in apredetermined rotation direction on positions corresponding to allpixels within the same region on the sample; and making total scanningtime the same, or increasing the total scanning time, or decreasing thetotal scanning time.
 2. The SEM image acquisition method according toclaim 1, further comprising: synthesizing a plurality of images of theposition corresponding to the same region and the same pixels on thesample from when the sample is scanned from different rotationdirections by the electron beam, on the basis of the signal detected andamplified.